Intervertebral disc degeneration causes a number of clinical problems relating to reduced disc height and herniation. In many cases, a simple discectomy can effectively relieve pain, but in time results in further collapse of the disc space because the intervertebral disc can no longer resist body loads the same as a healthy disc. Spine fusion procedures represent another state of the art treatment for disc problems. Fusion generally involves the use of interbody fusion cages and spinal fixation systems to immobilize the fusion site. However, it would be desirable to provide pain relief to the patient without substantially reducing the patient's range of motion.
In an effort to substantially maintain the patient's range of motion, the art has considered nucleus pulposus replacement and enhancement devices. Many of these devices are designed to fill at least a portion of the void left by removal of the nucleus pulposus portion of the disc and provide joint flexibility and shock absorption. Some of the nucleus pulposus devices being evaluated are in situ cured (such as in situ cured polyurethane contained within an outer bladder and in situ cured protein polymers). However, the fluid nature of these in situ cured materials provides the potential for these materials seep out of the disc space both intraoperatively and postoperatively. Other devices under evaluation include relatively solid hydrogels (such as hydrogel contained within a UHMWPE pillow and hydrogel balls). However, these hydrogel devices suffer from problems related to migration out of the disc space (expulsion) and subsidence.
US Published Patent Application No. 2002/0147497 (“Belef”) discloses a substantially flexible elongate body of fill material 412 which may be introduced through an opening in the annulus fibrosis. The body of fill material is fed through the opening until it substantially fills the interior of the disc. See FIG. 7 of Belef.
US Published Patent Application Number US 2002/0026244 (“Trieu”) discloses a first nucleus pulposus replacement device having a load-bearing elastic body surrounded by a deformable outer shell.
In a second embodiment, such as FIGS. 15 and 16, Trieu discloses a shape memory implant configured to form a spiral or other annular shape. These implants are deformable from a first folded state to a second substantially straight shape. In the first state, the device can be inserted into a disc space through a minimal access tube, and in a second state, the device can take on an annular shape in order to conform to the inner wall of the annulus fibrosus.
Although the devices of Trieu provide desirable load-bearing and minimal access qualities, they appear to transfer load through the center of the natural endplates. For example, in the first embodiment of Trieu, the middle of the device is completely filled with the inner elastic body. In the second embodiment, the inner hole is so small that the middle portion of the body is still substantially filled with the shape memory material. Because the middle portion of the natural endplates are typically weaker than the peripheral portions, transferring load to the central portion of the endplates may be mechanically problematic by causing adverse stress shielding of the annulus and possible endplate fracture.
In addition, the devices of Trieu may also be susceptible to slippage. For example, in the first embodiment of Trieu, the outer shell envelops the inner elastic body in all directions. If the outer shell moves laterally, there is nothing to prevent further lateral movement. In the second embodiment, the inner hole is fairly small and so any tissue growing therethrough may not stabilize the device.
In addition, because the disc changes shape during natural spinal movement, the Trieu devices will also deform. Lastly, because the ends of the spiral Trieu devices overlap, the Trieu device produces internal sliding interfaces. Natural spinal movement may produce wear debris at these internal sliding interfaces.